Process and apparatus for the removal of wire enamel insulation



March 19, 1968 R. H. SAVAGE 3,

PROCESS AND APPARATUS FOR THE REMOVAL OF WIRE ENAMEL INSULATION I Filed Jan. 51, 1964 Enameled Wire Fig].

Fig. 2.

Inventor Robert h! Savage, by a4 H/s A/forney.

United. States Patent Office 3,374,117 Patented Mar. 19, 1968 ABSTRACT OF THE DISCLOSURE Apparatus'es and processes are described for the rapid and effective removal of organic insulating film from the surface of an electrically conducting metallic wire by the controlled application of flameless heat thereto in the presence of oxygen in a longitudinally-extending walled cavity.

A first apparatus provides for the controlled separate but sequential admission of an oxidizing gas and a reducing gas to the same heated cavity. A second apparatus provides two heated cavities for sequential treatment of the wire, one of the heated cavities having oxidizlng gas supplied thereto and the other heated cavity having reducing gas supplied thereto. Means are disclosedfor automation of the sequence and timing of gas admission.

The differences in the processes depend upon the difference between the decomposition temperature of the organic enamel and the melting temperature of the metallic substrate. If this temperature difference is less than about 200 C., the wire enamel is subjected in sequence to the steps of coking, oxidizing and reducing, all conducted at elevated temperatures. If the temperature difference is greater than about 200 C., the coking step may be eliminated.

This invention relates to the removal of the insulating coverings from wires and more particularly to the removal of organic enamel film electrical insulation from the surface of wire electrical conductors.

Various processes have been employed to effect the removal of organic enamel film insulation from the surface of wire conductors (both single and multiple strands) for a short distance back from the end of the wire to enable electrically conducting anchorage of such wires to terminals as in small motors, transformers and lamp ballasts by way of example. Wires of relatively large diameters, i.e., larger than 0.020 inch in diameter may be satisfactorily stripped of insulation mechanically by such methods as wire brushing or brushing with a glass fiber mat. However, such methods are inadequate, or at least unsatisfactory, for the removal of wire enamel insulation from the surface of wires 0.020 inch in diameter or smaller, because such small diameter conductors are easily damaged by the use of mechanical abrading. The cost of dipping to effect chemical removal is relatively high, and chemical removal by dipping, with some types of enamels, is not effective.

It is, therefore, a principal object of this invention to provide a simple process and apparatus for the quick, effective and inexpensive removal of organic enamel insulating films from the surface of a substrate thread or rod.

It is another object of this invention to provide a process for the removal of organic enamel insulating films from the outer surface of an electrical wire conductor in a manner leaving a clean bright exposed length of wire conductor free of oxides to promote good electrical contact therewith.

A further object of this invention is the provision of apparatus conducive to automatic operation for reliably and safely removing organic enamel insulating films from fine wires.

The above objects may be secured by exposing an enamel-coated member to the simultaneous application of an elevated temperature and successive contact with a non-oxidizing atmosphere, an oxidizing atmosphere and a final exposure to a reducing atmosphere each preferably in the form of a sweep of gas. Briefly described, the

apparatus for the conduct of the above process comprises at least one plenum to receive the coated member therein and, as well, the entry of fluids thereto; means for heating this plenum to a high temperature and means for selectively admitting fluids there-to for the conduct of the above process. Means may also be included to automate the sequence of the admission of non-oxidizing, oxidizing and reducing fluids.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification in relation to the annexed drawing in which:

FIG. 1 is a schematic representation of a first embodiment of the apparatus of this invention wherein a single furnace is employed;

FIG. 2 is a schematic arrangement for the automatic sequence actuation for the control valve of FIG. 1; and

FIG. 3 is a second embodiment of a device for the practice of this invention wherein separate furnaces are employed for exposure to the non-oxidizing and oxidizing atmospheres.

The zone for high temperature exposure of the wire to be stripped in the presence of selected fluids is located in furnace 11 of the embodiment of FIG. 1. The ceramic or quartz furnace 11 is preferably in the shape of a tube with a flared mouth 12 to facilitate admission of the end ofwire 13 to the interior thereof. Although the dimensions of this tube are not critical, it is preferable to utilize a tube with a small bore to reduce the quantity of fluid employed. A bore size of from 2 to about 15 mm. represents a practical range of tube sizes for these devices. A short distance inward of open end 12 is the hot zone 14 extending along a sizeable length of the tube, which length is surrounded by wire heater 16 comprising a plurality of turns of a high resistance wire, as for example platinum or platinum-rhodium wire. Leads 17 and 18 may be of thicker silver wire welded to either end of the wire heater 16 and fused to the furnace tubing at least one point to provide anchorage. By passing current from transformer 19 through heater 16, zone 14 of furnace 11 is heated along its length to the desired temperature.

The maximum temperature in the hot zone is limited by the melting point of the metal of which the wire is composed. In the case of copper from which enamel insulation was removed in the examples provided herein the maximum temperature employed was about 930 C. In the case of silver, platinum or various alloy wire compositions the maximum temperature will vary with the particular wire composition, of course.

The necessary fluids for the conduct of the process are sequentially admitted to the interior of furnace 11 by way of end 20 to which is connected the conduit 21. Conduit 21 places the interior of furnace 11 in flow communication with the outlet of rotary valve 22 which, depending upon its angular position, will selectively pass fluids from source 23 and conduit 24 (non-oxidizing gas) or from source 26 and conduit 27 (oxidizing gas). Suitable shutoff valves 28 and 29 are supplied in conduits 24 and 27, respectively.

Automatic registry of rotary valve 22 for a selected flow sequence may be effected by an arrangement such as is shown schematically in FIG. 2. The operator may initiate the sequence of gas flow by means of a switch 31, which may be manually operated or foot-operated, whereby timer 32 is actuated. Timer 32 upon actuation supplies power to solenoid 33, which operates shut-off valve 34. Before valve 34 is opened by the movement of rod 36 into solenoid 33 whereby spring 37 is compressed, valve 22 is so positioned that conduit 27 is in communication with conduit 21 via passage 38 (FIG. 1) in valve 22. As soon as valve 34 is opened, non-oxidizing gas passes to furnace 11 at plenum pressure, that is, slightly above atmospheric pressure and continues for a preselected length of time. At the end of this flow period timer 32 supplies power to solenoid 39 causing actuation thereof. Actuation of solenoid 39 forces rod 41 outwardly compressing spring 42 and simultaneously turning rotary valve 22 through an arc of 120 in the clockwise direction by means of the cooperation of rack 43 and pinion 44, the latter being aflixed to valve 22 coaxially therewith. Stops 46 and 47 limit the travel of rod 41 thereby limiting the rotational movement of valve 22 to a 120 arc. Upon clockwise repositioning of valve 22, conduits 21 and 24 are placed into communication via passage 38 and an oxidizing gas is introduced to furnace 11 at a pressure slightly above atmospheric. At the end of the flow period designated by timer 32, solenoid 39 is deactivated and rod 41 is drawn back into solenoid 39 simultaneously rotating valve 22 in the counter-clockwise direction under the force of spring 42. Then for the extent of a third flow period the non-oxidizing gas is once more admitted to furnace 11 until timer 32 causes the supply of power to solenoid 33 to be cut off allowing spring 37 to force rod 36 and the valve controlled thereby to completely shut off the flow of any fluid into furnace 11. Thus, the valve 22 has been repositioned awaiting repetition of the above-described cycle.

All that is necessary for the conduct of the method then is for the operator to insert the end of wire 13 into hot zone 14 and close switch 31. Thereafter, in sequence in zone 14, which is constantly heated, the organic insulation covering the end of wire 13 will be submitted to a to a coking step (heating in a non-oxidizing atmosphere); submitted to rapid oxidation (heating in an oxidizing atmosphere) and finally reduced (heating in a reducing atmosphere.) Necessarily the non-oxidizing gas employed in the third step will contain an amount of reducing gas such as hydrogen. The preferred range for the concentration of hydrogen in nitrogen, for example is from 1% up to the ignition limit as determined by actual test, as by emitting the mixture into air in the presence of a spark or flame. Usually the non-oxidizing gas employed in the first step will contain an amount of reducing gas such as hydrogen to permit reuse of a single source. The latter step may, of course, be provided for by admitting the reducing gas from a third source of supply through valve 22 to furnace 11. Also, indicating means (not shown) may be provided in connection with valve 22 to inform the operator which fluid is currently being admitted to hot zone 14.

The preferred rates of gas flow in the first and third sweeps are those rates which overcome the diffusion of air into the tube from the open end 12 up to those rates which defeat the practicable heating of the zone 14 to the effective temperature range for the process.

By the use of apparatus disclosed herein it has been made possible to strip organic enamel insulation from fine wires quickly, without damage to the wire leaving the treated end reduced to the bright metal and free of oxides. A particularly novel aspect of this invention is the close temperature and reaction control provided whereby organic enamel insulations whose temperatures of decomposition approach to within about 70 C. of the melting point of the metal of which the wire is made can be successfully removed without melting, degrading or damaging the wire.

This feature is very important wi h. r p t to fine c pper wires, which are insulated with a film of a polyimide material, the nature and preparation of which is welldescribed in British Patent No. 903,271, published Aug. 15, 1962. Thus, during tests with polyimide-coated copper wire 0.010 inch diameter or smaller the decomposition temperature of the polyimide was determined to approach softening temperature of the copper. When these coated wires were heated in oxygen in an oven to a high temperature, approaching the softening temperature of copper, the insulation layer burst into flame and disappeared but in the process enough heat was generated during burning of the polyimide enamel to cause heating of the copper wire beyond its melting point thereby melting the copper wire within hot zone 14.

The preferred oxidizing atmosphere for the second sweep is air, rather than oxygen, for applications requiring the higher furnace temperatures, as for example in the case of polyimide high temperature insulation. This preference is based on the necessity of avoiding too rapid local heating of the wire by sudden oxidation of the enamel such as would occur in oxygen, for those applications wherein it is known that the wire will approach its softening temperature range during the stripping process, because of the high temperature of decomposition of the insulation.

By the use of a three-step process wherein an end portion of an enamel insulated wire is subjected to the continuous application of high temperature in hot zone 14 during successive exposures to sweeps of fluid providing in sequence a coking environment, an oxidizing environment and finally a reducing environment, a sufliciently fine degree of control is provided for the operator whereby the temperature of the reaction can be maintained below the melting point of the wire substrate but above the decomposition temperature of the enamel insulation to effect its removal. The entire sequence lasts but a few seconds and in each instance the wire end which extended into hot zone 14 emerges bright and clean without damage to the wire.

EXAMPLE 1 In a wire enamel stripping device of the type shown in FIG. 1 wherein a ceramic tube having a 5 millimeter bore and being about mm. in length was employed as furnance 11 with the heating zone 14 defined by about 38 turns of a platinum-rhodium wire covering 60 mm. of the central portion of furnace 11, polyimide-enameled copper wire has been stripped successfully without damage to the copper in repeated trials with slight variations in the temperature within heating zone 14. In a typical case, the end of a 0.010 inch diameter wire insulated with a film of polyimide material was introduced into hot zone 14 maintained at a temperature of about 830 C. by passing a current of from about 3.4 to about 3.6 amperes under about 30 volts D.C. While the end of the wire was held in hot zone 14, a sweep of dry nitrogen containing 12% hydrogen, which is well below the ignition concentration, was introduced into the interior of furnace 11 for a period of about 10 seconds. By repositioning the valve 22 the sweep of nitrogen-hydrogen was supplanted with a sweep of dry air supplied to the interior of furnace 11 for a period of about 5 seconds as in the aforementioned description. The valve 22 was once more repositioned to cut off the flow of oxygen and re-introduce a sweep of the nitrogen-hydrogen employed as the first sweep of gas. The flow of gas was kept to a minimum, about 2 cu. ft./hr. As has been noted hereinabove, the size of the furnace bore is relatively small, having a practical upper limit of about 180 square millimeters in area. Also, care was taken to place the end of the insulated wire in the central zone of the furnace bore away from the wall thereof. The presence of the excess nitrogen substantially eliminated any slight ignition which might otherwise have occurred as the result of the minor amount of mixing of hydrogen and oxygen which could have occurred in the series of gaseous replacements.

During the period of the first sweep of gas, coking of the polyimide insulation was indicated by the visible evolution of vapor therefrom during the nitrogen-hydrogen exposure. The rate of this vapor evolution indicated that the appropriate coking period was 7 seconds under the stated conditions but this "time decreased with increase of temperature. Upon introducing air to the hot zone 14, a rapid burning of the polyimide insulation decomposition products was detected. After the 5 second air exposure, a substantially immediate final reduction of the surface of the copper wire followed during the final sweep of the nitrogen-hydrogen mixture. After being removed from furnace 11, it was found that the copper wire in each instance was undamaged and cleanly stripped for lengths of from 10 to 30 mm.

Variations of the abovedescribed method may be employed, such as the use of pure dry hydrogen for the first and third sweeps; the use of a nitrogen-hydrogen mixture for the first sweep and the use of pure dry hydrogen for the third sweep.

Although the use of polyimide enamel coatings as insulation for fine wires has been increasing, in those applications in which the superior performance of the polyimide insulation is not required and in which the additional expenditure therefor cannot be justified, much of the output of enamel-coated wire employs oleoresinuous, polyester or polyvinyl formal insulation material. Such enamel-insulated wires are, of course, commercially in extensive use. A description of appropriate polyvinyl resins may be found in Jackson et a1. Patent 2,307,588, and Baszewski Patent 2,730,466; a description of the polyester resins may be found in Precopio et al. Patent 2,936,296.

Stripping tests performed with copper wire in sizes ranging from 1 to 10 mils has indicated that very effective stripping of wires coated with the aforementioned enamels, which decompose at a temperature far below the melting point of copper, is very quickly, effectively, and safely conducted in the aforementioned three-sweep method of removal.

EXAMPLE 2 A furnance of the general construction shown in FIG. 1 wherein quartz tubing having a 2 mm. bore and being several centimeters in length was employed with a section thereof wound with platinum wire of 10 mils diameter with a winding rate of about 5 turns per cm. The wire heater 16 was welded at its end to mil silver wire loops which, in turn, were fused to the quartz tubing to provide anchorage thereto with the silver wire loops extending outward for connection to the transformer 19. Heating of the platinum helix heater 16 was effected by passing about 5 amperes of current therethrough thereby raising the temperature within the hot zone to about 870 C. The end portion of a 9 mil diameter wire coated with polyester film insulation (or oleoresinous or polyvinyl formal) was inserted into hot zone 14. A sweep of dry hydrogen was passed into furnace 11 under a pressure slightly above atmospheric for a period of about 2 or 3 seconds during which time a small burst of smoke indicated the decomposition of the enamel by coking. lmmediately thereafter, hydrogen was replaced by a sweep of oxygen lasting 1 to 2 seconds during which time the residue, primarily carbon, was removed from the end of the wire. The oxygen was then cleared out of the tube with a sweep of dry hydrogen for a period of about 1 second after which the Wire was withdrawn. The entire process was completed within 6 seconds and a bright, clean end section was produced on the wire for a length approximately equal to the length of penetration into the hot zone 14.

Since the decomposition and removal of the lattermentioned low-temperature organic enamel insulations can be effected at temperatures far below the melting point of copper, it was determined that for stripping such insulations the above-described three-step process could be executed as a two-step process. Thus, with wires insulated with enamel insulations having decomposition temperatures at least 300 C. lower than the melting temperature of the wire substrate, stripping of the wire may be effected by subjecting the end of the wire to the continuous application of high temperature during which a sweep of an oxidizing fluid is employed followed by a sweep of a reducing fluid.

By way of illustration, this two-sweep method will be described in connection with the wire enamel stripper illustrated in FIG. 3, although it is to be understood that this two-sweep process may be conducted in the single furnace embodiment (FIG. 1). Likewise the three-sweep process may be conducted in the two furnace embodiment (FIG. 3). It is also to be noted, that although the practice of this invention is particularly advantageous with wire diameters of 0.020 inch or less, larger wires may nonetheless be successfully stripped in this manner.

However, conduct of the two-sweep process has indicated that the two-sweep procedure is unsuccessful for the stripping of wire insulated with enamel insulations having decomposition temperatures about 200 C. or less below the melting point of the wire substrate. This behavior is substantiated by the aforementioned oven tests with copper wire coated with polyimide enamels. Therein it was established that although the polyimide insulation burst into flame and disappeared at a temperature below melting point of the copper wire, sufficient heat was generated by the burning of the polyimide material to carry the temperature of the copper wire above its melting point thereby destroying the portion of the wire so heated.

In the embodiment of FIG. 3 an oxidizing fluid is supplied to quartz furnace 51 via conduit 52 and valve 53 from a source 54. A reducing environment is maintained in furnace 56 by introducing the reducing fluid via conduit 57 and valve 58 from the source 59 thereof, such as hydrogen. Power is supplied to heaters 61 and 62 from a volt supply employing a watt transformer 63 to reduce the voltage and a variable autotransformer 64 as a voltage control mechanism. Valves 53 and 58 may, of course, be automatically controlled; however, because of the very small volumes of gases employed in furnaces 51 and 56, it is suitable simply to allow the constant introduction of the gases at low flow rates so long as the furnaces 51 and 56 are spaced from each other at least 5 inches to minimize any possibility of creating an explosive mixture.

A wire to be stripped of organic enamel insulation having a low decomposition temperature may be inserted into furnace 51 sufficiently far to protrude into hot zone 64 where it is heated in the presence of an oxidizing fluid for a short period. The wire is then removed and inserted in a similar fashion into furnace 56 to protrude into hot zone 66 where the end of the wire is heated in a reducing atmosphere for a few seconds. The wire when removed from heater 56 has a clean, bright, stripped portion for about the length exposed to stripping conditions.

EXAMPLE 3 The end portion of a 0.010 inch diameter wire having a polyvinyl formal insulating film was inserted into hot zone 64 through which oxygen was flowing. A small burst of smoke indicated that the insulation was burned after an exposure time of about 2 seconds. The wire was removed from furnace 51 with the end thereof oxidized and darkened in color. The wire was then inserted into hot zone 66 where it was heated in the presence of dry hydrogen for about 2 seconds. During this short period, the hydrogen reduced the oxides present on the stripped portion of the wire and, when the wire was removed,

7 the stripped portion was clean and bright with the total time for stripping the wire being approximately 6 seconds.

The above-described sequence may be conducted equally well employing a nitrogen-hydrogen mixture in place of the hydrogen. It has been determined that in the case of wire sizes larger than 0.020 inch diameter, because of their greater mass, care must be taken to remove the wire from the reducing furnace 56 at a slow enough rate to allow sufficient cooling of the wire to prevent re-oxidization of the stripped portion. The temperature of the hot zones 64 and. 66 should be about 750-850 C. for the effective removal of oleoresinous, polyvinyl formal, and polyester insulation films.

In a more sophisticated construction than the simple operative devices disclosed herein, it may be desirable to provide a suitable exhaust mechanism for removing the small amount of smoke that develops during the stripping operation and perhaps anti-flashback means to isolate the sources of the sweeping fluids from the furnaces.

Therefore, as has been described herein, this invention provides a practical, safe, rapid and economical appartus and method for stripping organic enamel film-type insulation from wires in a manner enhancing the receptivity of the stripped portion of the wire to solder because of its oxide-free condition. Although this invention is particularly advantageous for use in connection with Wire having a diameter between 0.0001 and 0.010 inch,, the size range in which conventional methods such as wire brushing, brushing with glass fibers, and dipping in chemical solutions are either impractical, inoperative, or damaging to the wire substrate, this invention is nonetheless applicable to wire sizes in the range between 0.010 and 0.020 inch in diameter, particularly when removal from the reducing furnace is eifected at a slow enough rate to enable cooling of the wire to prevent its re-oxidization upon exposure to the atmosphere.

Although the device as ilustrated contemplates a stationary furnace or furnaces, it is readily apparent that conduit 21 may be flexible and the furnace with which it communicates may be properly insulated and made about the size of a large fountain pen. With such a construction the furnace may be moved into juxtaposition with the portion of the wire to be stripped either manually or with an automatic mechanism.

Various modifications are contemplated and may obviously be resorted to by those skilled in the art without departing from the spirit and scope of the invention as hereinafter defined by the appended claims, as only preferred embodiments thereof have been disclosed.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method for removing organic enamel insulation from a portion of insulated metallic wire comprising the steps of:

(a) controllably flamelessly heating the portion of wire to be cleaned in a non-oxidizing atmosphere at a temperature higher than the decomposition temperature of the organic enamel insulation and less than the melting point of the wire substrate, to therm'ally decompose said insulation,

(b) controllably flamelessly heating said portion in an oxidizing atmosphere at a temperature higher than the decomposition temperature of said organic enamle insulation and less than the melting point of the wire substrate to burn away any residue thereon of products of decomposition of said insulation, and

(c) flamelessly heating said portion in a reducing atmosphere.

2. The method substantially as recited in claim 1 wherein the organic enamel insulation has a decomposition temperature of less than about 200 C. below the melting temperature of the metal wire and the oxidizing atmosphere is air.

3. A method for removing organic enamel insulation 8 from a portion of insulated metallic wire comprising the steps of:

(a) flamelessly heating the portion of wire to be cleaned in a non-oxidizing atmosphere to thermally decompose said insulation,

(b) flamelessly heating said portion in an oxidizing atmosphere to burn away any residue thereon of products of decomposition of said insulation, and

(c) flamelessly heating said portion in a reducing atmosphere.

4. A method for removing organic enamel insulation from the surface of a portion of metallic wire covered with an organic enamel having a decomposition temperature of more than about 200 C. less than the melting temperature of the wire substrate, said method comprising the steps of:

(a) controllably flamelessly heating the portion of the wire to be cleaned in an oxidizing atmosphere at a temperature higher than the decomposition temperature of the insulation and less than the melting point of the wire substrate to decompose and burn away the insulation therefrom, and

(b) flamelessly heating said portion in a reducing atmosphere.

5. A method for removing organic enamel insulation from the surface of a portion of metallic wire covered with an organic enamel having a decomposition temperature of more than about 200 C. less than the melting temperature of the wire substrate, said method comprising the steps of:

(a) controllably flamelessly heating the portion of wire to be cleaned in an oxidizing atmosphere to decompose and burn away the insulation therefrom,

(b) flamelessly heating said portion in a reducing atmosphere, and

(c) permitting said portion to cool before withdrawal thereof from the reducing atmosphere.

6. In an apparatus for the removal of organic enamel insulation from the end of a metallic conductor by the controllable application of fiameless heat in the presence of oxygen to the end of said metallic conductor in a longitudinally-extending walled cavity having an open end whereby said end of said metallic conductor may be located therein, the improvement comprising:

(a) means connected to a wall of the heated cavity for controllably and selectively conducting gas flow to the interior of said heated cavity,

(b) a source of oxidizing gas connected to and in flow communication with said gas conducting means, and

(c) a source of reducing gas connected to and in flow communication with said gas conducting means.

7. The improvement in apparatus for removing organic enamel insulation from wire substantially as recited in claim 6 wherein the maximum cross-sectional area of the heated cavity is about 180 square millimeters.

8. The improvement in apparatus for removing organic enamel insulation from wire substantially as recited in claim 6 wherein the flamelessly heated wall cavity is a heating coil surrounding a ceramic tube.

9. The improvement substantially as recited in claim 6 wherein the gas conducting means includes an automatically timed valve in flow communication with a single conduit connected to a wall of said cavity.

10. In an apparatus for the removal of organic enamel insulation from the end of a metallic conductor by the controllable application of flameless heat in the presence of oxygen to the end of said metallic conductor in a longitudinally-extending walled cavity having an open end whereby said end of said metallic conductor may be located therein, the improvement comprising:

(a) first means connected to a wall of the heated cavity for controllably conducting gas flow to the interior of said heated cavity,

(b) a source of oxidizing gas connected to and in flow communication with said first gas conducting means,

9 (c) a second longitudinally-extending flamelesslyheated cavity mounted adjacent the walled cavity recited above at a distance of at least about 5 inches, (d) second means connected to a Wall of said second heated cavity for controllably conducting gas flow to the interior of said second heated cavity, and (e) a source of reducing gas connected to and in flow communication with said second gas conducting means. 11. The improvement in apparatus for removing organic enamel insulation from wire substantially as recited in claim 10 wherein the maximum cross-sectional area of each of the heated cavities is about 180 square millimeters.

References Cited UNITED STATES PATENTS MORRIS O. WOLK, Primary Examiner. G. R. MYERS, Assistant Examiner. 

